(1) Field of the Invention
The present invention relates to an optical element called “microlouver” by which the range of the exit direction of a transmitted light beam is restricted. Further, the present invention relates to an illuminating optical device and a display device as represented by a liquid crystal display (LCD) or a plasma display using such an optical element.
(2) Description of the Related Art
Liquid crystal displays are used as display devices for various electronic devices such as mobile phones, Personal Digital Assistance (PDA), ATM (Automatic Teller Machine) or personal computers, and recently there are practical uses for liquid crystal display that have a wide, viewable area.
A liquid crystal display, whose display area is visible from a wide range of angles, has been found to be useful when a plurality of people together look at a display screen. However, in the case certain devices, such mobile phones, in which it is assumed that personal privacy is desired, the wide, viewable screen may sometimes allow unwanted third parties to view displayed information, which will be displeasing to the user. Further, in the case of an information processing terminal used by an unspecified number of the general public, it is necessary to prevent other people from peeping at displayed information, when highly confidential information such as personal information is displayed. Therefore, there has been developed a liquid crystal display that can be switched between a display area that has a narrower field of view and a display area that has a wider field of view (see Japanese Patent Laid-Open No. 10-197844, the thirty-fifth paragraph).
FIG. 1 shows one example of a liquid crystal display capable of switching between a display form having a narrow field of view and a display form having a wide field of view. Referring to FIG. 1, the liquid crystal display has display panel 100 composed of a plurality of pixels arrayed in a matrix, and microlouver 101 to be attached onto this display panel 100. Microlouver 101, as shown in FIG. 2, is configured in a manner such a periodic structure body, in which light absorption layer 102 and transparent layer 103 are alternately disposed, is sandwiched between two protective films 104, and the period of positioning light absorption layer 102 and transparent layer 103 is constant. In a light beam incident on transparent layer 103, only a light beam whose incident angle is equal to or smaller than θ/2 (where θ is a visible angle) can pass through transparent layer 103. A light beam whose incident angle is larger than θ/2 is absorbed in light absorption layer 102. The visible angle θ is determined by a thickness D and a pitch P of the period of the periodic structure body. The smaller the visible angle θ, the higher is the directivity of a light beam which passes through microlouver 101.
In the display that has a narrow field of view, display panel 100 is used with microlouver 101 being attached to it. The region of a light beam visible from display panel 100 is restricted by microlouver 101. On the other hand, in the display that has a wide field of view, display panel 100 is used with microlouver 101 being removed. In this case, the visible region is determined by the visible angle of display panel 100. By using polymer dispersed liquid crystal for display panel 100, the visible region can be enlarged.
Further, there is a liquid crystal display having a built-in microlouver. FIG. 3 shows a configuration of the main portion of the liquid crystal display.
Referring to FIG. 3, the liquid crystal display includes backlight unit 200 and LCD panel 203 illuminated with a light beam from backlight unit 200, and between backlight unit 200 and LCD panel 203, microlouver 201 and diffuser 202 are disposed.
LCD panel 203 includes a plurality of liquid crystal cells arrayed in a matrix having a constant pitch, and R (red), G (green) and B (blue) color filters are positioned in a predetermined order so as to correspond to positions of the liquid crystal cells. FIG. 4 shows an example of positioning the color filters. In this example of positioning, color filters 203a are positioned in a matrix in a region divided by black matrix 203b for absorbing light, and a pitch thereof is constant.
Microlouver 201, as shown in FIG. 5, has a periodic structure in which light absorption layer 201a and transparent layer 201b are disposed alternately. Diffuser 202 is formed of polymer dispersed liquid crystals and adapted to be able to switch between a transparent state in which an incident light beam exits, as it is, and a scattered state in which the incident light beam exits as a diffused light beam due to scattering. For diffuser 202, there is, for example, PNLC (Polymer Network LC) or PDLC (Polymer Dispersed Liquid Crystal).
FIG. 6 shows a light beam at a narrow field of view. At the narrow field of view, diffuser 202 is made transparent. Microlouver 201 restricts the range of the exit direction of the diffused light beam from backlight unit 200. A light beam which passes through microlouver 201, as is, passes through diffuser 202 and illuminates LCD panel 203.
FIG. 7 shows a light beam at a wide field of view. At the wide field of view, diffuser 202 is made to scatter. Microlouver 201 restricts the range of the exit direction of the diffused light beam from backlight unit 200. The light beam which passes through microlouver 201 is formed into a diffused light beam by diffuser 202. LCD panel 203 is illuminated with this diffused light beam from diffuser 202.
However, display devices using the microlouver described above have the following problems.
In the liquid crystal display shown in FIG. 1, because both display panel 100 and microlouver 101 have a periodic structure, the light beam which passes through microlouver 101 is formed into a form in which two, regular intensity distributions based on their periodic structure are superimposed one on the other, thereby producing Moire fringes that correspond to a difference between their spatial frequencies.
In the liquid crystal display shown in FIG. 3, because both LCD panel 203 and microlouver 201 have a periodic structure, Moire fringes that correspond to a difference between spatial frequencies of their periodic structure are produced.
FIG. 8 shows a production principle of Moire fringes, FIG. 8A shows the spatial arrangement of a microlouver having a periodic structure. FIG. 8B shows a spatial frequency of the microlouver in the two-dimensional coordinate system. FIG. 8C shows a spatial arrangement of a display element having a periodic structure. FIG. 8D shows a spatial frequency of the display element in the two-dimensional coordinate system. FIG. 8E shows that the spatial frequencies shown in FIGS. 8B and 8D are superimposed one on the other in the two-dimensional coordinate system.
The spatial frequency as shown in FIG. 8A obtained by a two-dimensional Fourier transform of the spatial arrangement having a repetitive period (PI) in the one-dimensional direction, as shown in FIG. 8B, has a one-dimensional, regular peak arrangement (shown by a triangular mark).
Coordinates of the peak in the two-dimensional coordinate system are provided by integral multiplication of a vector PI (I*PI). The value of the vector PI is equal to the inverse of the period of the microlouver.
On the other hand, the display element as shown in FIG. 8C, in which pixels are formed in a matrix, has a spatial arrangement with a period in the x-direction (Px) and a period in the y-direction (Py). The spatial frequency obtained by a two-dimensional Fourier transform, as shown in FIG. 8D, has a two-dimensional, regular peak (shown by a circular mark). Coordinates of the peak in the two-dimensional coordinate system are provided by integral multiplication of a vector Px and a vector Py (n*Px+m*Py).
Superimposing the spatial frequencies shown in FIGS. 8B and 8D one on the other, the peak arrangement is represented by the relationship shown in FIG. 8E. Coordinates of each peak in the two-dimensional coordinate system are provided by integral multiplication of a vector PI, Px and Py (I*PI+n*Px+m*Py).
In FIG. 8D, ({vector Px}−{vector Py}) forms the bases of Moire fringes. Further, when a screen that includes a picture element having three pixels of R, G and B is used, Moire fringes are produced even under the condition of ({vector 3Px}−{vector PI}).
The problem concerning Moire fringes arises not only between a display element and a microlouver, but when components having periodicity are stacked one on top of the other. For example, Moire fringes are produced even between a lens sheet having a plurality of lenses disposed on its surface and a microlouver.